A conventional flash memory cell includes a gate stack, a source, a drain, and a channel disposed between the source and the drain. To form a conventional memory cell, a tunnel oxide is grown on a semiconductor substrate. Typically, the gate stack is then formed on the tunnel oxide. The gate stack is then exposed to an oxidizing agent at a high temperature to grow a layer of oxide on the gate stack. Once the growth of the oxide layer is completed, the source and drain are implanted.
The oxide layer is grown on the gate stack of the conventional memory cell for several purposes. One purpose of the oxide layer is round the corner of the floating gate. This rounding reduces electric fields which would otherwise be highly concentrated at the corner when the memory cell is biased, for example during an erase.
Although oxidizing the gate stack rounds the corner of the floating gate, the oxidation step also thickens the tunnel oxide just under the edge of the floating gate. As the corner of the floating gate rounds, the oxide on the surface of the silicon continues to grow. The tunnel oxide also continues to grow under the edges of the floating gate, resulting in a "bird's beak" shaped tunnel oxide near the edge of the floating gate.
The bird's beak shape of the tunnel oxide is undesirable for many reasons. For example, the bird's beak shaped tunnel oxide adversely affects erase and placement of the source. In order to erase the conventional memory cell, charge carriers tunnel from the floating gate to the source. Tunneling of charge carriers depends in part on the thickness of tunnel oxide through which the charge carriers must tunnel. To increase tunneling and raise erase efficiency, the tunnel oxide between the source and floating gate should be thin. Because of the bird's beak shape, the source is typically driven farther under the gate to reach a thinner portion of the tunnel oxide. Driving the source farther under the gate makes the channel smaller. As a result, short channel effects which degrade the performance of the memory cell, increase.
Accordingly, what is needed is a system and method for providing a memory cell having a reduced gate edge bird's beak. The present invention addresses such a need.